Power over Ethernet (PoE) (according to the IEEE 802.3-2005 standard) is used to provide power to network devices over network cabling. Typically, power is provided at up to about 50V at a current of up to about 600 mA (for various extensions to PoE, such as draft PoE+). In a typical PoE implementation (such as standard IEEE 802.3-2005), a maximum of 15.4 W is supplied per device (of which, generally, only 12.95 W is available for device use, the rest potentially dissipating along the cable). Thus, if the powered source equipment (PSE) is equipped with a 1500 W power supply, 97 powered devices (PDs) may be powered by the PSE.
In order to allow more low-powered devices, the devices may indicate a power class. PoE uses 5 power classes (0-4), with classes 0 and 3 providing up to 12.95 W to a PD, class 1 supporting up to 3.84 W, and class 2 providing up to 6.49 W. For example, a PD which uses less than 3.84 W may indicate a power class of 1. The 1500 W PSE can power a class 1 PD with only 4 W of power, thus enabling up to 375 class 1 devices to be powered. Because 12.95 W is insufficient for many complex devices, PoE+ (according to the draft IEEE 802.3at standard) (and other PoE extensions) was developed. PoE+, as currently envisioned, also allows finer granularity in power classification. Thus, a PoE+ PSE having a 1500 W power supply (which approaches the maximum that can be reliably delivered over a 20 A/115 VAC circuit) can power 50 video camera connections that use 30 W each (due to cable resistance, each camera actually only receives about 25.5 W).